Modern telecommunications networks such as cellular telephone networks can support a variety of advanced call-control services, such as call forwarding, pre-paid calling, adding or dropping call parties, or communicating pictures or video. For example, the Customised Applications for Mobile network Enhanced Logic (CAMEL) standard provides call-control services to users of second-generation (2G) and third-generation (3G) cellular networks such as Global System for Mobile Communications (GSM) networks or Universal Mobile Telecommunications System (UMTS) networks. More recently, fourth-generation (4G) cellular networks such as Long Term Evolution (LTE) access networks, interoperating with Internet Protocol (IP) Multimedia Subsystem (IMS) core networks, have begun to provide packet-switched (PS) voice and data connections. Such packet-switched connections can provide greater speed and throughput than do circuit-switched (CS) connections, and can make packet-switched data from other networks, such as the Internet, more readily available. Most 4G cellular networks, however, still additionally use access networks that provide circuit-switched connections, such as GSM or UMTS, due to the substantial infrastructure investment needed to expand packet-switched access networks. Such circuit-switched access networks may provide comparable or, at times, better speed and quality than packet-switched access networks for some types of data, including synchronous communications such as full-duplex voice communications.